Administration officials sought Monday to explain why so much less H1N1 flu vaccine is available than had been promised, blaming the manufacturers and the vagaries of science for nationwide shortages.

Public anxiety has surged as the swine flu sweeps across the country and doctors and clinics are forced to turn away many people. Confusion and frustration at immunization sites have increased the pressure on government officials and executives of the vaccine manufacturers to explain why optimistic pronouncements this summer about the vaccine's availability ended up so far off the mark.

In July, Obama administration officials said companies could make 80 million to 120 million doses by mid-October. They outlined an aggressive response to the pandemic, spending more than $2 billion to buy 250 million doses of vaccine and promising enough to inoculate every American.

But only about 16.5 million doses have become available so far, putting the administration in an uncomfortable political position regarding what President Obama declared last week to be a national emergency.

Health and Human Services Secretary Kathleen Sebelius said in television interviews Monday that officials had been "relying on the manufacturers to give us their numbers, and as soon as we got numbers we put them out to the public. It does appear now that those numbers were overly rosy."

Her deputy, Nicole Lurie, said in a separate interview with The Washington Post that when the companies "hit some stumbling blocks, they sometimes thought the fix was around the corner and didn't always feel the need to tell us, and then sometimes it turned out the fix wasn't around the corner."

Representatives of the companies said they kept the government informed along the way about challenges, including a slower-than-expected growth of the vaccine inside chicken eggs.

"We have a formal call with them once a week and are in touch with them probably on a daily basis," said Donna Cary, the top spokesperson for Sanofi Pasteur of Lyon, France. "We're pretty much right on track."

The administration's response has generally received high marks from public health experts and medical observers. Harvey V. Fineberg, president of the National Academy of Science's Institute of Medicine, said: "I think they've been fairly candid. They've always qualified their statements and made no excessive promises."

But remaining unsatisfied are some other medical experts and politicians whose constituents have been unable to get the vaccine. Local governments have been forced to shift vaccination campaigns and, in many cases, limit who is allowed to get the vaccine, at least for now.

In Fairfax County, for example, school officials announced a mass weekend immunization at its schools, only to cancel it and replace it with a more targeted clinic last weekend for pregnant women and kids younger than 3, citing limited quantities of the vaccine. On Monday, Fairfax immunized more than 1,000 people, but health workers said they were forced to turn others away.

Caught in the middle are parents who have heard contradictory messages and remain unable to get their children immunized.